The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) II. The radial structure of debris discs

TL;DR

The paper presents a comprehensive, high-resolution ALMA survey of 24 debris discs (ARKS) and a multi-method analysis to recover their radial dust distributions. By combining CLEAN imaging, non-parametric (frank, rave) and parametric modelling, the study identifies widespread substructures, including multi-ring systems and halos, and quantifies ring widths and edge slopes. The results show that many debris rings are significantly narrower than previously thought and that inner-edge steepness and outer-edge eccentricities encode dynamical histories possibly driven by planets, migration, or collisional evolution. The findings imply that some debris-disc architectures may be inherited from protoplanetary discs, but a sizeable subset requires later dynamical processing, with implications for planet populations at tens of au and the evolution of planetary systems. All radial profiles and models are made available for further dynamical modelling and comparative studies.

Abstract

The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) was recently completed to cover the lack of high-resolution observations of debris discs and to investigate the prevalence of substructures such as radial gaps and rings in a sample of 24 discs. This study characterises the radial structure of debris discs in the ARKS programme. To identify and quantify the disc substructures, we modelled all discs with a range of non-parametric and parametric approaches. We find that of the 24 discs in the sample, 5 host multiple rings, 7 are single rings that display halos or additional low-amplitude rings, and 12 are single rings with at most tentative evidence of additional substructures. The fractional ring widths that we measured are significantly narrower than previously derived values, and they follow a distribution similar to the fractional widths of individual rings resolved in protoplanetary discs. However, there exists a population of rings in debris discs that are significantly wider than those in protoplanetary discs. We also find that discs with steep inner edges consistent with planet sculpting tend to be found at smaller (<100 au) radii, while more radially extended discs tend to have shallower edges more consistent with collisional evolution. An overwhelming majority of discs have radial profiles well-described by either a double power law or double-Gaussian parametrisation. While our findings suggest that it may be possible for some debris discs to inherit their structures directly from protoplanetary discs, there exists a sizeable population of broad debris discs that cannot be explained in this way. Assuming that the distribution of millimetre dust reflects the distribution of planetesimals, mechanisms that cause rings in protoplanetary discs to migrate or debris discs to broaden soon after formation may be at play, possibly mediated by planetary migration or scattering.

Figures (17)

• Figure 1: Examples of the main radial functional forms from the parametric modelling code and their free parameters: double power law (a), triple power law (b), power law + error function (c), asymmetric Gaussian (d), double Gaussian (e), and double power law with Gaussian gap (f). The double Gaussian functional form is shown twice to demonstrate how it can model two rings or a ring with extended emission.
• Figure 2: Azimuthally averaged radial profiles derived from the CLEAN image (dotted black) and the deconvolved and deprojected surface brightness profiles fitted with frank (solid green), rave (dashed orange), and parametric models (dash-dotted red). The robust parameters used for CLEAN imaging are listed in Table \ref{['table:hyperparameters']}. The solid bars in the upper left corner of each panel indicate the FWHM resolved by frank if the radial profile were infinitesimally narrow (i.e. the frank PSF). The dotted bars indicate the synthesised beam size. In general, oscillations in the CLEAN profile that are likely due to noise (e.g. as seen in HD 15257 and HD 161868) are smoothed out in the frank, rave, and parametric models. The deconvolved peaks recovered by the frank, rave, and parametric models are often sharper than seen in the (beam-convolved) CLEAN images; the most extreme case is the best-fitting parametric model for HD 131488. We note that not all methods are applicable to every disc (e.g. azimuthal averaging was not applied to edge-on discs to extract the CLEAN profile, and frank was not applied to HD 39060 due to mosaicking being used in the observations). The offset between rave and frank for HD 107146 is due to the two methods fitting at different effective wavelengths.
• Figure 3: Deconvolved and deprojected surface density profiles fitted with frank, except for $\beta$ Pic (HD 39060) which was fitted with rave. These profiles are based on the surface brightness profiles presented in Fig. \ref{['fig:rp_brightness']}, but offers a cleaner and simpler alternative to that figure which includes a more comprehensive summary of the different approaches used to model the radial structure in this study. The bars in each panel are the same as those described in Fig. \ref{['fig:rp_brightness']}.
• Figure 4: Distribution of best functional forms for all targets, as determined by weighing the AIC confidence intervals, BIC differences, profile plots, and residuals (described in Sect. \ref{['sec:parametric_models']}).
• Figure 5: Comparison between the distribution of fractional widths found from the REASONS Matra2025 and ARKS datasets. The vertical axis shows the probability density function (PDF). Discs found to have multiple rings in ARKS are separated into their constituent rings. Two distributions are shown for REASONS, which correspond to the distribution of all REASONS discs, and the REASONS values for discs that overlap with those in ARKS respectively. REASONS discs with fractional uncertainties greater than 0.5 have been excluded from the REASONS distributions.